This invention relates to thermal transfer printers each having a thermal head and a platen, and more particularly it is concerned with a thermal printer capable of printing also on a paper of rough surface with a high quality.
As disclosed in Japanese Utility Model Unexamined Publication No. 34447/85 and Japanese Patent Unexamined Publication No. 94373/85, there are two types of thermal transfer printers: one type has a thermal head and a platen of cylindrical shape positioned against the thermal head, and the other type has a thermal head and a platen of flat plate shape. In view of the principles of thermal printing, these thermal printers are characterized in being able to print characters on a sheet of ordinary paper. Printing sheets used in the thermal transfer printers referred to hereinabove have a relatively smooth, flat surface. This is because the use of sheets of paper of rough surface which are low in smoothness suffers disadvantages in that ink melted by the heated thermal head cannot flow to depressed regions of the surface of the paper and only adheres to the elevated regions thereof, with the result that voids, omission of printed characters, a reduction in the darkness of printed characters and symbols and other defects appear on the transfer-printed sheet. However, with an increasing popularization of thermal transfer printers, demand for the use of paper of rough surface to provide transferprinted sheets of high quality to reduce costs is growing nowadays.
Meanwhile proposals have been made to use, as disclosed in Japanese Patent Unexamined Publication No. 76272/84, a thermal head having heat generating elements located at the crest of each arcuate glaze layer to improve transfer of thermal energy to the sheet of paper to effect thermal transfer-printing satisfactorily.
However, it would be impossible to accomplish the object of providing a thermal transfer-printed sheet of high quality by using paper of rough surface merely by improving transfer of thermal energy to the transferprinting sheet. Thermal heads of the prior art will be outlined by referring to FIGS. 1-4.
FIG. 1 shows in a schematic view a thermal head of the basic form used in a thermal transfer printer. As shown in the figures, a thermal head 1 comprises a substrate 2 formed of heat insulating material, such as ceramics, a plurality of glaze layers 3 on the substrate 2 extending lengthwise thereof and a plurality of heat generating elements 4 located on the crest of each glaze layer 3. Each glaze layer 3 has electrodes 5 and 6 connected to the heat generating elements 4, and a protective layer 7 for the heat generating elements 4 and electrodes 5 and 6. The electrodes 5 and 6 are connected to opposite sides or the left and right sides of the heat generating elements 4 to provide an opening A to define the size of transfer-printing dots. The protective layer 7 has the functions of preventing oxidation of the heat generating elements 4 and electrodes 5 and 6 which might otherwise be caused by their exposure to atmosphere and of avoiding wear which might otherwise be caused on the heat generating elements 4 and electrodes 5 and 6 as the thermal head travels on the surface of a sheet of paper or a transfer-printing film.
The dimensions of the glaze layers 3 may vary depending on the glass material used for their fabrication. Generally, however, their width w as measured transversely of the substrate is in the range between 800 and 1000 .mu.m and their height h as measured vertically of the substrate is in the range between 30 and 50 .mu.m. As is clearly seen, their radius of curvature r is about 2000 .mu.m which is very large, owing to the fact that the width w is disproportionately greater than the height h. Thus the crest of each glaze layer 3 is substantially flat or planar. Consequently, the opening A for the heat generating elements 4 at the crest of each glaze layer 3 is substantially planar, and the height H.sub.1 of the top surface of each heat generating element 4 as measured from the surface of the substrate 2 (which is also the case with heights H.sub.2, H.sub.3, H.sub.4 and h presently to be described) is smaller than the height H.sub.2 of the highest portions of the electrodes 5 and 6 by an amount substantially corresponding to the thickness (about 1-2 .mu.m) of the layer of electrodes: 5 and 6. The protective layer 7 is formed in uniform thickness on the electrodes 5 and the opening A for the heat generating elements 4 by vaporization deposition, so that the height H.sub.3 of the protective layer 7 at the opening A for the heat generating elements 4 is smaller than the height H.sub.4 of the protective layer 7 on the electrodes 5 and 6 by an amount substantially corresponding to the thickness of the layer of electrodes 5 and 6. Thus the opening A through which heat is released forms a recess which is lower in elevation than the surrounding area, with a result that difficulty is experienced in bringing a printing sheet into intimate contact with the thermal head. Because of this, a thermal transfer-printed sheet provided by using paper of rough surface would have a low quality. The reason for this phenomenon will be described by referring to FIGS. 3 and 4. FIG. 3 is a transverse sectional view showing the manner in which an inked ribbon 8 and a printing sheet 9 are brought into contact with the thermal head 1 in which the thermal head is of the prior art and the printing sheet is paper of rough surface on which characters and symbols are to be printed by thermal transfer-printing. FIG. 4 shows the distribution of contact surface pressure between the printing sheet 9 and thermal head 1 maintained in contact with each other as shown in FIG. 3. As shown in FIG. 3, the surface of the printing sheet 9 which is paper of rough surface is so low in flatness that elevated regions 9a and depressed regions 9b have a differences lying in the range between 10 and 28 .mu.m. The spacing interval or pitch p between the elevated regions 9a and depressed regions 9b is in the range between 80 and 300 .mu.m. Meanwhile the width d (see FIG. 2) of the opening A for the heat generating elements 4 may vary depending on the size of the dots for effecting transfer printing and is generally in the range between 120 and 180 .mu.m. As can be seen in the figures, the depressed regions of the printing sheet of rough surface have a great depth and the opening A for the heat generating elements 4 are recessed, so that it is impossible for the opening A to come into contact with the depressed regions 9b of the surface of the printing sheet 9 through the inked ribbon 8. Thus, even if the heat generating elements 4 generate heat to melt the ink spread on the inked ribbon 8, the melted ink could not adhere to the surface of the printing sheet 9 and voids and omission of printed characters would mar the surface of the transfer-printed sheet. To avoid this problem, it is necessary to raise the contact surface pressure between the thermal head and printing sheet to resiliently deform the printing sheet so as to temporarily render the surface of the printing sheet flat and smooth. However, when the thermal head 1 of the prior art is used, it is impossible to raise the contact surface pressure at the opening A for the heat generating elements 4, because the opening A at which the contact surface pressure should be raised is recessed and the protective layer for the electrodes 5 and 6 which has a greater height than the opening A is brought into contact with the printing sheet before the opening A is brought into contact therewith. An increase in the biasing force urging the thermal head into contact with the printing sheet would result in an increase in the contact surface pressure between the portion of the thermal head where the electrodes 5 and 6 are located. However, contact surface pressure at the opening A which is the most important location in the thermal head would not rise. As described hereinabove, the glaze layers 3 are substantially flat. Thus, even if the biasing force urging the thermal head to move is increased, the substrate of the thermal head having nothing to do with the printing operation would only be brought into contact with the printing sheet. After all, a biasing force of tremendous magnitude would be required to achieve a necessary increase in the contact surface pressure between the opening A and the printing sheet. Experiments conducted by us show that the biasing force would have to have a value of about 2 kg which is beyond the power of a thermal transfer printer of a mechanism now being produced for practical use to develop. Any modification of the mechanism would involve an increase in the number of parts and production costs and would not be economically viable.
Also, proposals have been made to improve the platen as disclosed in Japanese Utility Model Unexamined Publication No. 32043/85, for example. However, this improvement is intended to avoid a lopsided contact between the thermal head and platen, and the improvement is not directed to modifying the platen in a manner to allow thermal transfer-printing to be successfully effected on a sheet of paper of rough surface.